There is a conventional system of transmitting isochronous data having high real-time characteristics such as audio data or video data. This data transmission system can be utilized in a security system having plural monitoring cameras and one display device, where the security system composes video data from the respective cameras so as to display video on the one display device. Alternatively, the conventional data transmission system can be utilized in a rear-looking system which is mounted on a motor vehicle having cameras which take images on the right, left, and rear sides of the motor vehicle as substitutes for the right and left side mirrors of the motor vehicle, such a rear-looking system composes video data from the cameras so as to display video on a monitor in the vehicle.
FIG. 30 is a diagram illustrating an example of the prior art data transmission system.
The data transmission system 3000 as shown in FIG. 30 comprises a first source device (transmission device) 3002 for receiving a video data 3012 from a first data output unit 3101, which is external to the system, as a first isochronous data Dt1 and transmitting the isochronous data Dt1 at a predetermined timing, and a second source device (transmission device) 3003 for receiving a video data 3103 from a second data output unit 3102, which is external to the system, as a second isochronous data Dt2 and transmitting the isochronous data Dt2 at a predetermined timing The data output units 3101 and 3102 are image-taking devices such as CCD (charge coupled device) cameras. The first source device 3002 has a transmission buffer 3022 which retains the isochronous data Dt1 for a prescribed period, i.e., from its input timing to its output timing. The second source device 3003 has a transmission buffer 3023 which retains the isochronous data Dt2 for a prescribed period, i.e., from its input timing to its output timing.
The data transmission system 3000 comprises a sink device 3001 which is connected to the respective source devices 3002 and 3003 via a transmission line 3000a such as a data bus. The sink device 3001 receives the isochronous data that is transmitted by the respective source devices 3002 and 3003 and outputs the isochronous data at a predetermined timing. The data transmission system 3000 also comprises a video data buffer 3030 for containing an isochronous data 3011 that is output by the sink device 3001 and outputting the contained isochronous data to a video composition device 3200, which is external to the data transmission system 300, at an output timing corresponding to the video composition device 3200. The sink device 3001 has a receiving buffer 3021 which retains the received isochronous data Dt1 and Dt2 for a prescribed period, i.e., from its receiving time to its output timing.
In addition, the data transmission system 3000 can transmit/receive asynchronous data, other than the isochronous data, such as command data for controlling the respective devices, between the sink device 3001 and the respective source devices 3002 and 3003 via the transmission line 3000a. 
As shown in FIG. 30, the data transmission system 3000 consisting of the first and second source devices 3002 and 3003, which are mutually connected by the transmission line 3000a such as a data bus, and the sink device 3001 is called a multimedia LAN (Local Area Network) or multimedia bus.
The operation of the data transmission system 3000 will now be described.
Hereinafter, as a specific operation of the data transmission system 3000, a description is given of a case where the first and second video data 3012 and 3013 that are output from the data output units (cameras) 3101 and 3102, which are external to the data transmission system 3000, are transmitted by the data transmission system 3000 to the video composition device 3200 which is external to the system, and then composition and display of the video data 3012 and 3013 is performed by the video composition device 3200.
To be specific, the video data 3012 and 3013 which have been output from the data output units 3101 and 3102 that are external to the system are input to the corresponding source devices 3002 and 3003 in the system 3000, respectively.
In the source device 3002, the input video data 3012 is retained in the transmission buffer 3022, and thereafter, the video data 3012 is output to the transmission line 3000a as the isochronous data Dt1 at a predetermined timing. In the source device 3003, the input video data 3013 is retained in the transmission buffer 3023, and thereafter, the video data 3013 is output to the transmission line 3000a as the isochronous data Dt2 at a predetermined timing.
Then, in the sink device 3001, the isochronous data Dt1 and Dt2 which have been transmitted from the source devices 3002 and 3003 to the transmission line 3000a at the predetermined timing are received, and the received isochronous data Dt1 and Dt2 are retained in the receiving buffer 3021.
Thereafter, in the sink device 3001, a processing of outputting, as a sink output 3011, the isochronous data Dt1 and Dt2 that are retained in the receiving buffer 3021 to the video data buffer 3030 at the prescribed timing is performed. At this time, the isochronous data Dt1 and Dt2 are output in the order in which the data are received by the sink device 3001.
In the video data buffer 3030, a processing for delaying one of the isochronous data Dt1 and Dt2 which have been input as the sink outputs 3011 is performed so that the output timings of the both data for the respective frames coincide with each other. The isochronous data Dt1 and Dt2 to be composed as data which form the same frame are output from the buffer 3030 to the external video composition device 3200 at the same timing.
In this video composition device 3200, a composed image of video that is taken by the respective cameras is displayed based on composed video data which is obtained by the composition processing for the isochronous data Dt1 and Dt2.
In the above-mentioned prior art transmission system 3000, the first and second source devices 3002 and 3003 transmit the video data (isochronous data) to the sink device 3001 at their own timings, respectively. Therefore, in the sink device 3001, the isochronous data Dt1 and Dt2 that are transmitted from the respective source devices are received at arbitrary timings and are stored in the transmission buffer 3021. Accordingly, the output timing corresponding to each frame of the isochronous data Dt1 which is output from the transmission buffer 3021 as the buffer output 3011 does not match the output timing corresponding to each frame of the isochronous data Dt2 which is output from the transmission buffer 3021 as the buffer output 3011.
Thus, in the prior art transmission system 3000, the video data buffer 3030 for containing the buffer output 3011 is provided in the subsequent stage of the transmission buffer 3021. Then, in the video data buffer 3030, the gap between the output timing corresponding to each frame of the isochronous data Dt1 and the output timing corresponding to each frame of the isochronous data Dt2 is absorbed.
However, in the video data buffer 3030, the isochronous data is delayed so as to absorb the gap between the respective output timings of the isochronous data that are outputted from the sink device 3001. Consequently, a data delay occurs from the time when the sink device 3001 receives plural video data as isochronous data to the time when the video data are synchronized in a frame unit and are output from the video data buffer 3030.
Hereinafter, problems due to the data delay are described in more detail by taking, as an example, a motor vehicle in which plural cameras for assisting the sense of sight as substitutes for back mirrors and the data transmission system 3000 are mounted, and video data from the plural cameras, as the substitutes for the back mirrors, are transmitted by the system so as to display composed images of these data on a vehicle-mounted monitor.
In this case, video images of backward views of the motor vehicle are displayed on the vehicle-mounted monitor and are past (delayed) images which have been delayed by a timing corresponding to the data delay in the sink device 3001, as compared to real time images that are taken by the camera. In the case of motor vehicles, it is not permissible to display the past images which have been delayed even a little from the real time on the monitor as the substitutes for the back mirrors. Therefore, the prior art data transmission system 3000 has a poor practicality as the data transmission system which is to be utilized in a system in which data delay is not permissible, such as the above-mentioned vehicle-mounted rear looking system.
Further, in the prior art data transmission system 3000, the video data buffer 3030 is required to synchronize the video data that is output from the sink device 3001, corresponding to the respective data output units, in frame units.